I guess this is more a problem with single character games, as with parties you can always have the 'charismatic' member who does all the talking.
IMHO Charisma, to make it clear it's not a 'dump stat', should be evident that it hurts not having it. How could you do that?
1) Not reward characters for combat, but for achievements or successes. You should spend in-game valuables during fights (be it potions, wand charges, time, whatever), more than what you get if you choose to fight, that way you really don't want to fight if you don't need to.
2) Have charisma a way of succeeding some 'encounters'. This, with point 1, means you achieved the same result, gained the same exp, but spent less resources.
3) Have some better rewards with high charisma. Maybe the reward for killing the dragon is a +2 sword, but with charisma you might convince the Count that it's in his best interest to reward you with his personal +3 sword.
4) Make the differences noticeable. I.E. receiving 200g instead of 225g is not really noticeable, remember you are choosing to increase your charisma 1 point instead of your strength, there should be obvious reasons why you would even consider doing it. The first quest you see that rewards you with a 'less awesome' weapon because you're insulting the quest giver should make you think.